Relative symlinks don't work

[yaychris]

On 0.1.6, Sprig would correctly load a seed file that's a symlink to a relative path (e.g. ln -s ../shared/animals.yml animals.yml).

0.2.0 no longer works:

Errno::ENOENT: No such file or directory @ rb_sysopen - ../shared/animals.yml

This commit appears to be the culprit. readlink only reads the filename of the symlink reference, which in this case, is ../shared/animals.yml. It's passed to File.open, which tries to open the filename relative to the current working directory, which doesn't work.

realpath should be used because it interprets the symlink relative to the directory of the symlink. PR forthcoming.